Numerous types of optoelectronic devices that are based on semi-conductor devices are known. For example, solar cells, light emitting diodes (LEDs), imaging devices, and photo-detectors may be based on semi-conductor devices. Such optoelectronic devices may be operated by forward biasing, reverse biasing, or no biasing. For example, an LED may be operated by forward biasing a diode junction to cause light to be emitted. Some photo-detectors or imaging devices may be operated by reverse biasing a diode junction (note that the diode junction may not need to be reverse biased for detector operation). Solar cells, otherwise known as photo-voltaic cells, may be operated without applying any bias. Instead, photons absorbed by photo-active regions of the device generate voltage or/and current.
The foregoing illustrates that such optoelectronic devices need two electrical contacts. For many optoelectronic devices, the electrical contacts are placed at opposite sides of the device, which may be referred to as a top and bottom electrical contact, respectively. Moreover, at least one side of the optoelectronic device needs to be able to either receive or emit light, at least for a relevant range of wavelengths. By a relevant range of wavelengths it is meant a range of wavelengths that is either absorbed or emitted by the photo-active regions.
For many optoelectronic devices, light is emitted or received through the top electrical contact. For such optoelectronic devices, a trade-off is made between optical transparency and electrical conductivity of the top electrical contact. For example, the top electrical contact may be made from a material that has good optical transparency, at least in the relevant range of wavelengths for the device. As one example, the top electrical contact may be formed from indium-tin-oxide (ITO). However, optically transparent materials may not be as electrically conductive as desired. Hence, operation of the device may suffer. For example, when being operated as a solar cell, greater resistance of the top electrical contact results in greater voltage drop across the top electrical contact. On the other hand, electrically conductive materials may not be as optically transparent as desired.
Note that some materials that may be used for top electrical contacts are optically transparent for a relatively narrow range of wavelengths. Therefore, they may be suited for optoelectronic devices that are intended to be operated over a narrow range of wavelengths. However, it may be desirable to operate the device over a wider range of wavelengths. For example, it may be desirable to operate a solar cell over a wide range of wavelengths in order to capture energy present across a wide range the solar spectrum. It may be desirable to operate some LED devices over a wide range of wavelengths also. For example, it may be desirable to have an LED device that can produce light of different colors. Also, imaging devices should be able to detect different colors.
Therefore, improvements are desired in the electrical contacts for optoelectronic devices.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.